How much shampoo do you use?

My question is how much shampoo do you use when applying? My girlfriend says the norm is a quarter size amount, but how much exactly is a quarter size? Can you eyeball a quarter amount in your hand and get the thickness the same as a quarter or does it just mean the diameter of a quarter? How much shampoo do you apply in reference to a quarter size; is it based on diameter, thickness, or both?